Solid electrolyte ion transport membranes appear to have significant potential for the separation of oxygen from gas streams containing oxygen. Of particular interest are mixed conductor materials that conduct both oxygen ions and electrons and which can hence be operated in a pressure driven mode without the use of external electrodes. Composite ceramic mixed conductor membranes comprised of multi-phase mixtures of an electronically-conductive material and an oxygen ion conductive material are disclosed by T. J. Mazanec et al. in U.S. Pat. No. 5,306,411 for electrochemical reactors and partial oxidation reactions. M. Liu et al. disclose in U.S. Pat. No. 5,478,444 composite mixed conductor materials containing oxygen-ion-conducting materials such as bismuth oxide and electronically conductive materials. True mixed conductors, exemplified by perovskites such as La.sub.0.2 Sr.sub.0.8 CoO.sub.x, La.sub.0.2 Sr.sub.0.8 FeO.sub.x, La.sub.0.2 Sr.sub.0.8 Fe.sub.0.8 Co.sub.0.1 Cr.sub.0.1 O.sub.x and others, are materials that possess intrinsic conductivity for both electrons and ions. Some of these materials possess among the highest oxygen ion conductivity known and also rapid surface exchange kinetics.
Although there is great potential for these materials in gas separation applications, there are some drawbacks in their use. A common problem of most ceramics is their fragility and low mechanical strength in tension which makes it difficult to fabricate large elements such as tubes and deploy them in high reliability commercial systems.
Yamamoto et al. reported the microcracking in sintered LaCoO.sub.x in Perovskite-Type Oxides as Oxygen Electrodes for High Temperature Oxide Fuel Cells, Solid State Ionics 22:241-46 (1987). Such microcracks are probably related to structural transformations during sintering and frequently occur in perovskites. The vacancy concentration in many ion transport membrane materials such as perovskites is a function of the oxygen partial pressure in the gas surrounding it. Since the unit cell size is dependent on the vacancy concentration, in many ion transport membrane materials the volume of the unit cell increase as P.sub.O2 is reduced. For example, in perovskites the size of the unit cell ABO.sub.3 increases as the partial pressure of oxygen on the anode or permeate side is reduced. The change is unit cell size gives rise to a compositional coefficient of expansion in addition to the thermal coefficient of expansion. Compositional gradients in the materials hence give rise to mechanical stress which could result in failure. This often necessitates close control of atmosphere during startup, shutdown operation or processing.
B. Fu et al. reported in (Y.sub.1-x Ca.sub.x)FeO.sub.3 : A Potential Cathode Material for Solid Oxide Fuel Cells, Proc. 3rd Intl. Symp. on Solid Oxide Fuel Cells, S. C. Singhal, Ed., The Electrochem. Soc. Vol. 93-4: 276-282 (1993) that microcracking was a problem in Y.sub.1-x (Sr or Ca).sub.x MnO.sub.3 perovskites due to either a high temperature polymorphic symmetry change, or a highly anisotropic thermal expansion coefficients of the system. Efforts made to circumvent this problem were unsuccessful. They found that high Sr or Ca doping levels (x&gt;0.3) can reduce the microcracking, but resulted in poor performance for fuel cell cathode applications apparently due to inferior physical properties including reduced ionic conductivities.
Because of the difficulties described above, the manufacture of perovskite tubes has required complex, carefully controlled process steps sometimes involving a sophisticated, controlled atmosphere sintering process. This increases the cost and complexity of fabrication and could result in problems during transient operation of the manufactured elements involving temperature, atmospheric or compositional changes.
It is highly desirable to minimize the sensitivity of the ion transport membrane to the ambient atmosphere. Also, many of these materials have very high thermal expansion coefficient (for example, La.sub.1-x Sr.sub.x CoO.sub.3 is approximately 20.times.10.sup.-6 /.degree. C.), which gives rise to high thermal stress during the processing and operation, hence often results in the failure of materials.
In summary, the use of the composite mixed conductors in the prior art is mostly confined to materials comprising multi-phase mixtures of oxygen ion conductors and electronic conductors. The sole objective in the prior art was to introduce electronic conductivity into the ionic conductor. In general this requires the electronically conductive second phase to be present in greater than 30% to 35% by volume when randomly distributed to enable operation above the percolation limit.
Composite bulk superconducting materials have been synthesized to achieve desirable physical properties and high T.sub.c superconducting characteristics. Wong et al. in U.S. Pat. No. 5,470,821 describe composite bulk superconducting materials with continuous superconducting ceramic and elemental metal matrices. The elemental metal is situated within the interstices between the crystalline grains to increase transport current density.